1. Field of the Invention
The present invention relates to a sensor system for tracking ground based vehicles, and more particularly, to a passive infra-red sensor system which is used in conjunction with Intelligent Vehicle Highway Systems to determine traffic information including the location, number, weight, axle loading, speed and acceleration of the vehicles that are in the field of view. In addition, the infra-red sensor system can be utilized to obtain information on adverse weather situations, to determine the emissions content of the vehicles, and to determine if a vehicle is being driven in a reckless manner by measuring its lateral acceleration.
2. Discussion of the Prior Art
The loss in productivity and time from traffic congestion as well as the problems caused by excess pollution are a significant drain on the economy of the United States. The solution, the management of ground based vehicular traffic, is becoming an increasingly complex problem in todays mobile society, but one that must be addressed. The goal of traffic management is to provide for the efficient and safe utilization of the nation's roads and highway systems. To achieve this simple goal of efficiency and safety, a variety of traditional sensor systems have been utilized to monitor and ultimately control traffic flow. Any traffic monitoring system requires a sensor or sensors of some kind. There are two general categories of sensors, intrusive and non-intrusive. Intrusive sensors require modification of, and interference with, existing systems. An example of a system incorporating intrusive sensors is a loop detector, which requires installation in the pavement. Non-intrusive sensors are generally based on more advanced technology, like radar based systems, and do not require road work and pavement modification. Within each of the two general categories, there are two further types of sensors, active and passive. Active sensors emit signals that are detected and analyzed. Radar systems are an example of systems utilizing active sensors. Radar based systems emit microwave frequency signals and measure the Doppler shift between the signal reflected off the object of interest and the transmitted signal. Given the current concern with electro-magnetic interference/electro-magnetic fields, EMI/EMF, and its effect on the human body, there is a general sense that the use of active sensors will be limited. Passive sensors are generally based upon some type of image detection, either video or infra-red, pressure related detection such as fiber optics, or magnetic detection such as loop detectors.
The loop detector has been used for more than forty years, and is currently the sensor most widely used for traffic detection and monitoring. The loop detector is a simple device wherein a wire loop is built into the pavement at predetermined locations. The magnetic field generated by a vehicle as it passes over the loop induces a current in the wire loop. The current induced in the wire loop is then processed and information regarding traffic flow and density is calculated from this data. Although loop detectors are the most widely used systems for traffic detection, it is more because they have been the only reliable technology available for the job, until recently, rather than the technology of choice. In addition, a significant drawback of the loop detectors is that when a loop detector fails or requires maintenance, lane closure is required to effect repairs. Given that the goal of these systems is to promote efficiency, and eliminate lane closure for maintenance and repair, loop detectors present a less than ideal solution.
A second common type of traffic sensor is closed circuit television. Closed circuit television (CCTV) has been in wide use for verification of incidents at specific locations, including intersections and highway on-ramps. Although CCTV provides the system operator with a good quality visual image in the absence of precipitation or fog, they are not able to provide the data required to efficiently manage traffic. The CCTV based system also represents additional drawbacks in that it requires labor intensive operation. One system operator can not efficiently monitor hundreds of video screens, no matter how well trained.
An advanced application which stems from the CCTV based system is video imaging. Video imaging uses the CCTV as a sensor, and from the CCTV output is able to derive data from the video image by breaking the image into pixel areas. Using this technology, it is possible to determine lane occupancy, vehicle speed, vehicle type, and thereby calculate traffic density. One video camera can now cover one four-way intersection, or six lanes of traffic. However, a drawback to video imaging is that it is impacted by inclement weather. For example, rain, snow or the like cause interference with the image. There are currently several companies that are marketing video imaging systems. Some of these systems are based upon the WINDOWS.TM. graphical user interface, while other companies have developed proprietary graphic user interfaces. All of these systems are fairly new, so there is not a wealth of long term data to support their overall accuracy and reliability.
As an alternative to video imaging, active infra-red detectors are utilized. Active infra-red detectors emit a signal that is detected on the opposite side of the road or highway. This signal is very directional, and is emitted at an angle to allow for height detection. The length of time a vehicle is in the detection area also allows for the active infra-red detector system to calculate vehicle length. Using this data, an active infra-red detector system is able to determine lane occupancy and vehicle type and calculate vehicle speed and traffic density. Additionally, over the distances that a typical highway sensor will observe, typically a maximum of approximately three hundred yards, active infra-red detectors are not hampered by the inclement weather over which video imaging systems fail to operate. However, in a multiple lane environment, due to detector placement on the opposite side of the road from the emitter, there can be a masking of vehicles if the two vehicles are in the detection area at the same time.